BackgroundDown syndrome (DS) is distinguished by cognitive disability, a concave profile, and systemic complications. Oral diseases have been reported to be common in DS patients.ObjectiveTo investigate the association between DS and periodontal diseases.MethodsTwo independent reviewers searched six bibliographic databases up to January 2023 and used additional search methods to identify published studies on gingivitis or periodontitis in people with and without DS. Meta‐analysis, risk of bias, sensibility analysis, publication bias, and evidence grading were all carried out.ResultsTwenty‐six studies were included for analysis. There was a tendency for increased plaque accumulation, periodontal probing, periodontal attachment level, bleeding on probing and indices in DS individuals. Meta‐analysis of 11 studies showed a significant association between DS and periodontitis (OR 3.93; 95% CI 1.81–8.53). Probing depth was significantly high in individuals with DS as compared to controls (mean difference 0.40 mm; 95% CI 0.09–0.70). Gingivitis was significantly associated (OR 1.93; 95% CI 1.09–3.41) with DS in four studies. The evidence was classified as ‘moderate certainty’.ConclusionMedium/low‐quality studies demonstrate that Down syndrome is strongly associated with periodontitis and moderately associated with gingivitis.